We present a method for imaging the optical near-fields of nanostructures, which is based on the local ablation of a smooth silicon substrate by means of a single, femtosecond laser pulse. At those locations, where the field enhancement due to a nanostructure is large, substrate material is removed. The resulting topography, imaged by scanning electron or atomic force microscopy, thus reflects the intensity distribution caused by the nanostructure at the substrate surface. With this method one avoids a possible distortion of the field distribution due to the presence of a probe tip, and reaches a resolution of a few nanometers. Several examples for the optical near-field patterns of dielectric and metallic nanostructures are given. The optical properties of nanostructures are an important issue in nanoscience with a tremendous potential for applications. This holds both for individual particles and particle arrays, and for dielectric materials (e.g., photonic crystals) as well as metals (e.g., optical antennas).1-3 The optical nearfields of such particles, which are essential for understanding their function, are not easily accessible by experimental means. One approach is to use a scanning near-field optical microscope to image the intensity distribution in the vicinity of the nanostructures with a fine aperture.4 Different approaches have improved the resolution to below 25 nm. 5,6We introduce here an alternative method that consists in imaging optical near-field intensities by means of intense short laser pulses. This technique also reaches a resolution of a few nanometers, much smaller than the laser wavelength used, but which, moreover, is not hampered by possible distortions of the resulting patterns due to the presence of a probe tip.In our experiments the nanoparticles, located on a smooth substrate, are irradiated with a single, femtosecond laser pulse, which is perpendicularly incident onto the substrate plane. The intensity of the pulse is adjusted to a value sufficiently low that the parts of the substrate far away from the particles are not affected. Nevertheless, the substrate surface under and near a particle will be ablated if the local intensity enhancement in the optical near-field is high enough.7 Thus, the resulting ablation pattern in the substrate, which can be imaged by scanning electron or atomic force microscopy (AFM), represents a nonlinear "photograph" of the optical near-field intensity distribution of the nanoparticle under study. Because of the short duration of the laser pulse, a smearing out of the resulting structures due to thermal conduction, as it can appear for nanosecond pulses, does not occur.Our investigations cover optical near-fields of both dielectric and metallic nanostructures. In the case of dielectric nanoparticles, we have used polystyrene spheres, available as monodisperse colloidal suspensions, with different diameters in the range of a few hundred nanometers. These particles were deposited on a silicon substrate (commercial silicon wafer) by means of spin coatin...

We compare simulations of optical near-fields of single triangular nanostructures with experimental results from a near-field ablation technique on a periodic arrangement of triangles. We find good agreement of the lateral near-field distributions; nevertheless their dependency on the polarization of the incident light differs by 90 • . Upon increasing the lateral distances of the nanotriangle arrangement in the experiment, the polarization dependence agrees with the simulation. We conclude that this at first sight unexpected behaviour stems from the coupling of near-fields by scattered surface waves and their interaction with the incoming beam. The ongoing miniaturisation is one of the driving forces for the field of nanotechnology. In that field it would be useful to have all kinds of optics available that are frequently used on a micrometer scale, e.g., spectroscopy but also exposition of photoresists. For that purpose light must be focussed to a tiny spot well below the diffraction limit. To achieve this goal different types of nanoantennas have been proposed and examined during the last years [1][2][3][4][5][6]. For a further improvement of the efficiency of these nanoantennas progress has to be made on both sides, the theoretical description of these nanoantennas and the imaging of the near-fields with highest resolution [6][7][8][9][10]. The SNOM (scanning optical near-field microscopy) is an instrument used frequently, but alternative imaging methods have been developed as well which do not involve the problem of interaction of the SNOMtip with the optical near-field [11][12][13][14][15][16]. Here, we study experimental simple realizable nanoantennas, Au-triangles produced by colloidal lithography, and compare near-field simulations with the experimental results of a near-field ablation technique [14], where the optical near-field is used to pattern a silicon surface. SimulationThe optical properties have been investigated by using the discrete dipole approximation (DDA) method for u Fax: +49-7531-883127, E-mail: johannes.boneberg@uni-konstanz.de solving for the scattering of light from nanostructures. Details of this method have been described previously [17,18]. In the present application, we focus on the field intensity distribution, as defined in terms of the the square of the local electric field, |E| 2 , in studies of a gold triangle on a silicon substrate. Dielectric constants for gold and for silicon are taken from Johnson and Christy [19] and Palik [20], respectively. In this calculation, the particle is discretized using a grid spacing of 3 nm for 240 nm edge length and 3.75 nm for 380 nm edge size, and the silicon substrate is treated using an effective medium approximation that uses a weighted average dielectric constant, where the weight factor is determined by the area of the particle that is exposed to the silicon. Past studies [21] have suggested that this level of discretization is capable of determining the qualitative shape of the intensity distribution in the near-field regio...

No abstract